Photoluminescence Spectra Correlations with Structural Distortion in Eu3+- and Ce3+-Doped Y3Al5-2x(Mg,Ge)xO12 (x = 0, 1, 2) Garnet Phosphors

Garnet-type materials consisting of Y3Al5-2x(Mg,Ge)xO12 (x = 0, 1, 2), combined with Eu3+ or Ce3+ activator ions, were prepared by a solid-state method to determine the structural and optical correlations. The structure of Y3Al5-2x(Mg,Ge)xO12 (x = 1, 2) was determined to be a cubic unit cell (Ia-3d), which contains an 8-coordinated Y3+ site with octahedral (Mg,Al)O6 and tetrahedral (Al,Ge)O4 polyhedra, using synchrotron powder X-ray diffraction. When Eu3+ or Ce3+ ions were substituted for the Y3+ site in the Y3Al5-2x(Mg,Ge)xO12 host lattices, the emission spectra showed a decrease in the magnetic dipole f-f Eu3+ transition and a redshift of the d-f Ce3+ transition, related to centrosymmetry and crystal field splitting, respectively. These changes were monitored according to the increase in Mg2+ and Ge4+ contents. The dodecahedral and octahedral edge sharing was identified as a key distortion factor for the structure-correlated luminescence in the Eu3+/Ce3+-doped Y3Al5-2x(Mg,Ge)xO12 garnet phosphors.


Introduction
Ce 3+ -doped Y 3 Al 5 O 12 (YAG) phosphor has been widely utilized as a smart light source in conjunction with blue LED chips [1][2][3].Initially developed in 1967 by G. Blasse and A. Bril, the yellow Ce 3+ -activated YAG phosphor was prepared for use in flying-spot cathode-ray tubes for color television, emitting intense yellow light via the 5d-4f transition of Ce 3+ ions within the cubic garnet YAG structure [4].The garnet mineral belongs to the nesosilicate subclass, characterized by isolated tetragonal polyhedra [5].The YAG garnet structure, a cubic crystal system (Ia-3d), consists of dodecahedral YO 8 , octahedral AlO 6 , and tetrahedral AlO 4 units.The local dodecahedral YO 8 polyhedra within the garnet structure exhibit edge-sharing with YO 8 and AlO 6 polyhedra, as well as vertex-sharing with isolated AlO 4 tetrahedra [6][7][8][9].In garnet host lattices, Ce 3+ activator ions can occupy the dodecahedral site, influencing crystal field splitting and resulting in a shift of the d-f transition [10][11][12].Researchers J. Ueda and S. Tanabe have explored the effects of crystal and electronic structures on Ce 3+ -doped YAG, particularly regarding the crystal field splitting of the lowest 5d levels and a new distortion parameter estimated by the ratio of dodecahedral edges [6].Similarly, Eu 3+ activator ions provide insight into site-resolved luminescence, distinguishing between centrosymmetric and non-centrosymmetric sites in garnet-type phosphors using the magnetic dipole and electric dipole transitions observed in emission spectra [13][14][15].In this study, the structure of single-phase Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1, 2) garnet materials was determined using synchrotron X-ray analysis, revealing a cubic unit cell with Ia3d symmetry.The cell parameters, volume, and distances within the host lattices were discussed.By substituting Ce 3+ or Eu 3+ activator ions into Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2), correlations between emission spectra, dipole transitions, and crystal field splitting were investigated in relation to structural distortion parameters.

Results and Discussion
Figure 1a,b illustrate the Rietveld refinement fitting of the powdered X-ray diffraction (XRD) data for Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1 and 2).The summarized structural data are provided in Tables 1-3.A single phase of the garnet structure, determined to be a cubic unit cell (Ia3d), was obtained through a solid-state reaction method.This cubic phase of garnet, including Y 3 Al 5 O 12 (YAG, ICSD 170157) and Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1 and 2) structures, comprises 8-, 6-, and 4-coordinated Y 3+ , Al 3+ (1)-Mg 2+ , and Al 3+ (2)-Ge 4+ ions, respectively, occupying 24c, 16a, and 24d Wyckoff sites.The 6-coordinated Mg 2+ (with a radius of 0.72 Å for 6 coordination number (CN)), Al 3+ (1) (with a radius of 0.535 Å for 6 CN), and 4-coordinated Al 3+ (with a radius of 0.39 Å for 4 CN) and Ge 4+ (with a radius of 0.39 Å for 4 CN) sites are suitable for substitutions in the garnet structure [19].Therefore, the formula for the garnet-structured Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) can be expressed as Y 3 Al(1) 2-x Mg x Al(2) 3-x Ge x O 12 based on the ionic radii of the cations in the unit cell, as depicted in Figure 1c.Similar to the isolated AlO 4 tetrahedra in the YAG structure, the tetrahedral (Al,Ge)O 4 polyhedron in the Y 3 Al 5-2x (Mg,Ge) x O 12 structures is also isolated, with no sharing of O atoms.There are two different bond distances between Y 3+ and O 2− ions in the YO 8 polyhedron, whereas a single bond distance is observed in the (Mg,Al)O 6 and (Al,Ge)O 4 polyhedra within the host lattices.The YO 8 polyhedron shares edges with nearby YO 8 polyhedra and (Mg,Al)O 6 octahedra.YAG exhibits lattice parameters and volume, such as a = 11.9900(14) Å and V = 1723.68Å 3 (as shown in Table 1).The cell parameters and volumes of Y 3 Al 5-2x (Mg,Ge Table 1.Rietveld refinement and crystal data for Y3Al5-2x(Mg,Ge)xO12 (x = 0, 1, 2).

Chemical Formula Y3Al5O12 (ICSD 170157) Y3MgAl3GeO12 Y3Mg2AlGe2O12
Radiation type, λ   In Figure 2a,b, a distinct shift in the apparent peaks to lower angles, particularly those at 2θ = 32-34 • , was observed as Al 3+ ions were gradually replaced by Mg 2+ and Ge 4+ ions in the Eu 3+ -and Ce 3+ -doped Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) phosphors, respectively.A single phase of the garnet structure with a cubic crystal system (Ia3d) was obtained, free from any apparent impurities.The cell volumes of both Y 2.5 Eu 0.5 Al 5-2x (Mg,Ge) x O 12 and Y 2.95 Ce 0.05 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) phosphors increased as the content of Mg 2+ and Ge 4+ increased.Figure 3a displays the emission photoluminescence (PL) spectra of Y 2.5 Eu 0.5 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) phosphors.The electronic f -f transitions of Eu 3+ ions in the host lattices are assigned as 5 D 0 -7 F 1 , 5 D 0 -7 F 2 , 5 D 0 -7 F 3 , and 5 D 0 -7 F 4 within the range of 550 and 750 nm [13][14][15].It is known that when Eu 3+ ions are located at the centrosymmetric site in a crystal structure, the magnetic dipole transition ( 5 D 0 -7 F 1 ) dominates, whereas, in the absence of inversion Eu 3+ ions in the host lattice, the electric dipole transition ( 5 D 0 -7 F 2 ) dominates [13][14][15].The centrosymmetric symmetry of the local-environmentcenter Eu 3+ ions in the Y 3 Al 5-2x (Mg,Ge) x O 12 structure was inferred from the normalized intensity ratio of the magnetic dipole transition, as depicted in Figures 3a, S1 and S2.All intensity values in the emission spectra were normalized by dividing them by the maximum intensity value of the electric dipole transition peak.As Eu 3+ ions occupy the 8-coordinated Y 3+ site in the Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) structures, the dominant magnetic dipole transition around 590 nm was noticeably decreased up to x = 0, 1, and 2. This indicates that the ideal cubic field, characterized by a centrosymmetric Y 3+ center (D 4h point group), was gradually distorted to a dodecahedral field with the substitution of Mg 2+ and Ge 4+ ions.Figure 3b presents the emission spectra of Y 2.95 Ce 0.05 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) phosphors (S1 and 2) and relative energy diagrams of cubic and distorted cubic polyhedrons, showcasing the 8-coordinated site geometry for the 5d Ce 3+ orbital energy levels in the host materials.Upon substituting Mg 2+ and Ge 4+ ions for Al 3+ ions in the structure, the distorted 8-coordinated Ce 3+ ions in the YAG exhibit a significant redshift of the e g orbital splitting from a normal cubic polyhedron, resulting in emissions from the red-shifted 5d energy level caused by the high-crystal-field effect [10][11][12].
sence of inversion Eu 3+ ions in the host lattice, the electric dipole transition ( 5 D0-7 F2) dominates [13][14][15].The centrosymmetric symmetry of the local-environment-center Eu 3+ ions in the Y3Al5-2x(Mg,Ge)xO12 structure was inferred from the normalized intensity ratio of the magnetic dipole transition, as depicted in Figures 3a, S1, and S2.All intensity values in the emission spectra were normalized by dividing them by the maximum intensity value of the electric dipole transition peak.As Eu 3+ ions occupy the 8-coordinated Y 3+ site in the Y3Al5-2x(Mg,Ge)xO12 (x = 0, 1, 2) structures, the dominant magnetic dipole transition around 590 nm was noticeably decreased up to x = 0, 1, and 2. This indicates that the ideal cubic field, characterized by a centrosymmetric Y 3+ center (D4h point group), was gradually distorted to a dodecahedral field with the substitution of Mg 2+ and Ge 4+ ions.Figure 3b presents the emission spectra of Y2.95Ce0.05Al5-2x(Mg,Ge)xO12(x = 0, 1, 2) phosphors (S1 and 2) and relative energy diagrams of cubic and distorted cubic polyhedrons, showcasing the 8-coordinated site geometry for the 5d Ce 3+ orbital energy levels in the host materials.Upon substituting Mg 2+ and Ge 4+ ions for Al 3+ ions in the structure, the distorted 8-coordinated Ce 3+ ions in the YAG exhibit a significant redshift of the eg orbital splitting from a normal cubic polyhedron, resulting in emissions from the red-shifted 5d energy level caused by the high-crystal-field effect [10][11][12].In Figure 4a, the unit cell and local structure of the dodecahedral site in Y 3 Al 5-2x (Mg,Ge) x O 12 are depicted.Ce 3+ ions engaged in the Y 3+ site of garnet structures can emit light due to the distortion factor expressed by the ratio of dodecahedral edges.The four short and long Y(Ce)-O bond distances in the garnet structures can be influenced by the deviation of O-O bonds shared with adjacent dodecahedra (d 88 ) and two non-shared tetrahedra (d 81 ) [6].This deviation in the local structural arrangement around the Ce 3+ ions within the garnet lattice can have an impact on the energy levels of the Ce 3+ ions [6].As distortion increases due to the compression on the cube, the maximum emission shifts to longer wavelengths according to the lower excited state of energy level in the cubic crystal field splitting.

Figure 1 .
Figure1a,b illustrate the Rietveld refinement fitting of the powdered X-ray diffraction (XRD) data for Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1 and 2).The summarized structural data are provided in Tables1-3.A single phase of the garnet structure, determined to be a cubic unit cell (Ia3d), was obtained through a solid-state reaction method.This cubic phase of garnet, including Y 3 Al 5 O 12 (YAG, ICSD 170157) and Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1 and 2) structures, comprises 8-, 6-, and 4-coordinated Y 3+ , Al3+ (1)-Mg 2+ , and Al 3+ (2)-Ge 4+ ions, respectively, occupying 24c, 16a, and 24d Wyckoff sites.The 6-coordinated Mg 2+ (with a radius of 0.72 Å for 6 coordination number (CN)), Al3+ (1) (with a radius of 0.535 Å for 6 CN), and 4-coordinated Al 3+ (with a radius of 0.39 Å for 4 CN) and Ge 4+ (with a radius of 0.39 Å for 4 CN) sites are suitable for substitutions in the garnet structure[19].Therefore, the formula for the garnet-structured Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 0, 1, 2) can be expressed as Y 3 Al(1) 2-x Mg x Al(2) 3-x Ge x O 12 based on the ionic radii of the cations in the unit cell, as depicted in Figure1c.Similar to the isolated AlO 4 tetrahedra in the YAG structure, the tetrahedral (Al,Ge)O 4 polyhedron in the Y 3 Al 5-2x (Mg,Ge) x O 12 structures is also isolated, with no sharing of O atoms.There are two different bond distances between Y 3+ and O 2− ions in the YO 8 polyhedron, whereas a single bond distance is observed in the (Mg,Al)O 6 and (Al,Ge)O 4 polyhedra within the host lattices.The YO 8 polyhedron shares edges with nearby YO 8 polyhedra and (Mg,Al)O 6 octahedra.YAG exhibits lattice parameters and volume, such as a = 11.9900(14)Å and V = 1723.68Å 3 (as shown in Table1).The cell parameters and volumes of Y 3 Al 5-2x (Mg,Ge) x O 12 (x = 1 and 2) are larger than those of YAG compounds, such as Y 3 MgAl 3 GeO 12 (a = 12.1479(2) Å and V = 1796.82(17)Å 3 ) and Y 3 Mg 2 AlGe 2 O 12 (a = 12.2628(1) Å and V = 1844.027(14)Å 3 ).The Y-O bond distances of an 8-coordinated Y (with a radius of 1.019 Å) comprise four long distances (2.433 Å) and four short distances (2.303 Å) in the YAG structure.The Y-O bond distances in Y 3 MgAl 3 GeO 12 and Y 3 Mg 2 AlGe 2 O 12 host lattices remain consistent with those of the YAG structure, as shown in Figure 1c.However, the bond distances of a 6-coordinated (Mg,Al)-O 6 and Mg-O 6 exhibit distinct increases from 1.921 Å (for Al-O in YAG) to 1.997 Å and 2.094 Å, respectively, representing 4% and 9% differences.Conversely, the bond distances of Al-O 4 and (Al,Ge)-O 4 tetrahedra in the garnet structures remain similar, ranging from 1.766 to 1.785 Å and 1.760 Å.Interestingly, with the increase in the cell parameter and volume, the bond distance Lattice parameter (Å) a = 11.9900(14) a = 12.1479 (2) a = 12.2628 (1)

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